In recent years, as vehicles are increased in a size, tires which can endure a heavy load are required. Usually, tires are equipped with a belt layer formed by laminating plural layers comprising steel cords and coating rubber for coating the steel cords to provide the tires with load resistance, traction resistance and the like. In heavy load vehicles, however, separation at a belt end is liable to be brought about in tires provided with the above belt layer. The separation at a belt end is brought about by fatigue of the rubber at a belt end which is caused by dynamic interlayer shearing strain brought about by a load, driving force, braking force and lateral force each applied on the tire in addition to interlayer shearing strain exerted by air pressure of the tire.
In order to prevent the separation at a belt end, it is devised to change various mechanical structures of the belt layer and a rubber component formulation thereof, and it is proposed to enhance heat separation resistance and cut separation resistance of the tire while maintaining load resistance, traction resistance and cracking resistance thereof.
Proposed is, for example, a radial tire in which 5 or more belt layers are employed and in which a specific coating rubber is used for a master layer and a protective layer to thereby improve heat separation resistance and cut separation resistance of the whole part of the tire (patent document 1). Further, proposed is a radial tire for a heavy load which is improved in separation resistance while maintaining abrasion resistance, traction resistance and cutting resistance by constituting a belt layer from four layers having different widths, inclining steel cords of the constituted layers toward an equatorial plane of the tire, setting an inclining direction of steel cords of an adjacent layer to an equivalent direction or an inverse direction and constituting the belt layers in which the respective layers have suitably different lengths in a tire width direction (patent document 2 and patent document 3). Further, proposed is a pneumatic radial tire for a heavy load in which a right and left one pair of belt end cushion rubbers are arranged at both end parts between at least two layers among rubber-coated cord layers forming a principal cross belt, in which at least two cushion rubbers among the cushion rubbers positioned at the same end part side are arranged overlapping partially on each other in a tire width direction and in which a deviation width of the overlapped cushion rubbers in a tire width direction falls in a specific range (patent document 4).
Also, proposed as well are a rubber composition which maintains high elasticity even in vulcanization of a large-sized tire for a long time and which has durability and a radial tire for heavy vehicles which is provided with a belt layer comprising the rubber composition and which has heat build-up resistance, durability and cracking resistance. That is, it is a radial tire provided with a belt layer in which a rubber composition compounded with trans-polybutadiene and N,N′-diphenylmethanebismaleimide as a heat resistant cross-linking agent in a specific proportion in combination is used for a coating rubber of steel cords (patent document 5).    Patent document 1: Japanese Patent Application Laid-Open Hei 7 No. 32815    Patent document 2: Japanese Patent Application Laid-Open Hei 5 No. 8607    Patent document 3: Japanese Patent Application Laid-Open Hei 6 No. 127213    Patent document 4: Japanese Patent Application Laid-Open Hei 11 No. 321224    Patent document 5: Japanese Patent Application Laid-Open No. 2003-63205